Mission Log: Apr. 9, 2010

By George SedberryGray's Reef National Marine Sanctuary

After an 18-hour steam from Port Canaveral, Florida, we arrived on station at 0615 EDT at a site about 100 nautical miles east of Sapelo Island, Georgia. We began our first acoustic surveys while waiting for sunrise. The ship’s sonar system enables us to map the bottom and detect rugged bottom and coral mounds with acoustics, and it can also detect fish in the water above. The time around sunrise and sunset is thought to be the best time for fisheries acoustic surveys of reef fish, as the large predatory species of interest are most active at those times. Large fishes living on deep coral reefs are also thought to be on the move around sunrise and sunset, and chances of detecting them moving above the reef is thought to be best then.

The ROV crew prepares the vehicle for launch.

Although we did not see any fish marks on the sonar returns on this first survey, we found some rough bottom that looked like it would support corals, and prepared to launch the Remotely Operated Vehicle (ROV) to look for corals visually. Unfortunately, the conditions prevented the launch. The Gulf Stream is flowing unusually strongly this week, and we had a 4-knot (4 nautical miles per hour) current that prevented us from lowering the ROV to the bottom. It also prevented us from lowering any other kind of sampling gear. The 35-knot winds and 8-ft seas did not help much either.

The flow of the Gulf Stream is indicated on this satellite sea-surface temperature (SST) image. The warm water of the Gulf Stream is colored red in the image. We attempted our first ROV dives along the western edge (*), but the currents were too strong, as they were to the west and southwest of this site. Click here for a larger version.

We hoped to find slower currents in deeper water farther offshore, but the currents were just as strong there in the middle of the Gulf Stream as they were along the inshore Gulf Stream front. We tried areas to the south, but the conditions were no better. We did find some very high-relief bottom habitat known to be productive wreckfishing grounds, and conducted sonar surveys over them to further map the extent of rugged bottom topography and to note any fish seen on the sonar. Schools of fish were seen “stacked up” at the edge of steep dropoffs.

After cruising around to several sites looking for the right combination of depth, current and hard bottom for ROV operations, we determined that we needed to move north of the Gulf Stream front. The Gulf Stream flow seems to be particularly strong during this cruise, so we decided to move further to the north and inshore to get out of the flow. There are areas of known hard bottom and corals on the upper continental slope just north of Charleston, South Carolina, and the PISCES will head there to attempt ROV transects tomorrow.

A screen capture from the fisheries acoustics system aboard the PISCES. The image depicts sonar signals from two sound frequencies (18 kHz top; 38 kHz bottom). The image depicts the bottom trace (red) as hard bottom, with a school of fish stacked up to the left of a steep incline, and another school rising from the bottom and forming an arc over an elevated plateau.